The need for installation of special control circuits in automotive vehicles is increasing. Typical of the type of control circuits presently in automotive vehicles include so-called anti-lock braking system (ABS) which prevent the locking of the vehicle wheels during braking, ASR Systems, which ensure correct traction of motor vehicles, devices for detecting speeds such as tachometers and devices for measuring the distance travelled such as odometers. At present these detection systems are comprised of basic component such as phonic rings, detecting sensors, on-board computers and hydraulic systems. The phonic ring is usually a serrated ring mounted on the rotating part desired to be monitored and the sensor comprising a passive or active type is mounted on the fixed part of the frame and confronts the serrated zone of the phonic ring at a predetermined distance. Electronic signals emanating from individual sensors are transmitted to the on board computer which in the case of the ABS computes the differences in the speed of wheels of the vehicle.
Passive sensors currently utilized operate by variation in reluctance and require no input. However, sensors of this type must be mounted externally to the bearing and are thus unprotected against possible impacts, contaminating agents and other hazards. This type of sensor is particularly sensitive to metal particles resulting from wear of the brakes near the sensor themselves and to the elevated temperatures developed by disc brakes. They also require appropriate calibration during the mounting phase by the user. It has also been found that the sensors are incapable of detecting rotational speeds down to zero miles per hour and thus cannot be use for tachometric and odometric measurements.